Physical layer protocol data unit format including padding in a high efficiency wireless lan

ABSTRACT

The present invention relates to a Physical layer Protocol Data unit (PPDU) format including a padding. According to one aspect of the present invention, a method for transmitting data to a plurality of stations on a transmission channel by an Access Point in a WLAN may be provided. The transmission channel may be divided into a plurality of subchannels allocated to the plurality of stations. The method may include generating a padding having a length individually for each of one or more subchannels to which paddings are applied among the plurality of subchannels, the length of the padding making transmissions end simultaneously on the plurality of subchannels, and transmitting a PPDU frame including a data unit without the padding or a data unit added with the padding for each of the plurality of subchannels to the plurality of stations on the transmission channel.

This application claims the benefit of Korean Patent Application No.10-2014-0108178, filed on Aug. 20, 2014, which is hereby incorporated byreference as if fully set forth herein. This application claims thebenefit of U.S. Provisional Application No. 62/146,899, filed on Apr.13, 2015, which is hereby incorporated by reference as if fully setforth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Wireless Local Area Network (WLAN),and more particularly, to a Physical layer Protocol Data Unit (PPDU)format including a padding in a High Efficiency WLAN (HEW), atransmitting method, receiving method, transmitting apparatus, receivingapparatus, and software using the PPDU format, and a recording mediumthat stores the software.

2. Discussion of the Related Art

Along with the recent development of information and telecommunicationtechnology, various wireless communication techniques have beendeveloped. Among them, the WLAN enables a user to wirelessly access theInternet based on radio frequency technology in a home, an office, or aspecific service area using a portable terminal such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), a smartphone, etc.

To overcome limitations in communication speed that the WLAN faces, therecent technical standards have introduced a system that increases thespeed, reliability, and coverage of a wireless network. For example, theInstitute of Electrical and Electronics Engineers (IEEE) 802.11nstandard has introduced Multiple Input Multiple Output (MIMO) that isimplemented using multiple antennas at both a transmitter and a receiverin order to support High Throughput (HT) at a data processing rate of upto 540 Mbps, minimize transmission errors, and optimize data rates.

As more and more devices such as smartphones have recently supportedWLAN, more Access Points (APs) have been deployed to support this trend.Although WLAN devices with high performance conforming to the Instituteof Electrical and Electronics Engineers (IEEE) 802.11ac standard areused more than legacy WLAN devices conforming to the IEEE 802.11g/nstandard, a WLAN system having higher performance is required due toWLAN device users' increased use of large-capacity content such as anUltra High Definition (UHD) video. A legacy WLAN system seeks toincrease a bandwidth and a peak transmission rate, only to fail toincrease performance that users actually feel.

HEW standardization is under discussion in a so-called IEEE 80.111x taskgroup. The HEW aims to increase performance felt by users demandinglarge-capacity, high-rate services, while supporting simultaneous accessof many Stations (STAs) in an environment in which a plurality of APsare densely deployed and the coverage of APs is overlapped.

However, there is no specified method for applying a padding to a PPDUframe format in a HEW.

SUMMARY OF THE INVENTION

Objects of the present invention is to provide a Physical layer ProtocolData Unit (PPDU) format including a padding in a High Efficiency WLAN(HEW), and a method and apparatus for transmitting and receiving signalsusing the PPDU format.

The objects of the present invention are not limited to the foregoingdescriptions, and additional objects will become apparent to thosehaving ordinary skill in the pertinent art to the present inventionbased upon the following descriptions.

In an aspect of the present invention, a method for transmitting data toa plurality of Stations (STAs) on a transmission channel by an AccessPoint (AP) in a WLAN may be provided. The transmission channel may bedivided into a plurality of subchannels allocated to the plurality ofSTAs. The method may include generating a padding individually for oneor more subchannels to which paddings are applied among the plurality ofsubchannels, wherein the padding has a length such that transmissionsend simultaneously on the plurality of subchannels, and transmitting aPPDU frame including a data unit without the padding or a data unitadded with the padding for each of the plurality of subchannels to theplurality of STAs on the transmission channel.

In another aspect of the present invention, a method for transmittingdata to an AP by an STA in a WLAN may be provided. The method mayinclude receiving a trigger frame from the AP, the trigger frameallocating a plurality of subchannels to the STA and one or more otherSTAs, generating, when a padding is applied to a subchannel allocated tothe STA, the padding having a length such that a transmission on thesubchannel allocated to the STA and transmissions on one or more othersubchannels allocated to the one or more other STAs end simultaneously,and transmitting a PPDU frame including a data unit without the paddingor a data unit added with the padding to the AP on the subchannelallocated to the STA.

In another aspect of the present invention, an AP apparatus fortransmitting data to a plurality of STAs on a transmission channel in aWLAN may be provided. The transmission channel may be divided into aplurality of subchannels allocated to the plurality of STAs. The APapparatus may include a baseband processor, a Radio Frequency (RF)transceiver, a memory, etc. The baseband processor may be configured togenerate a padding individually for one or more subchannels to whichpaddings are applied among the plurality of subchannels, wherein thepadding has a length such that transmissions end simultaneously on theplurality of subchannels, and to transmit a PPDU frame including a dataunit without the padding or a data unit added with the padding for eachof the plurality of subchannels to the plurality of STAs on thetransmission channel using the RF transceiver.

In another aspect of the present invention, an STA apparatus fortransmitting data to an AP in a WLAN may be provided. The STA apparatusmay include a baseband processor, an RF transceiver, a memory, etc. Thebaseband processor may be configured to receive a trigger frame from theAP, the trigger frame allocating a plurality of subchannels to the STAand one or more other STAs using the RF transceiver, to generate, when apadding is applied to a subchannel allocated to the STA, the paddinghaving a length such that a transmission on the subchannel allocated tothe STA and transmissions on one or more other subchannels allocated tothe one or more other STAs end simultaneously, and to transmit a PPDUframe including a data unit without the padding or a data unit addedwith the padding to the AP on the subchannel allocated to the STA usingthe RF transceiver.

In another aspect of the present invention, a software orcomputer-readable medium having instructions executable for an APapparatus to transmit data to a plurality of STAs on a transmissionchannel in a WLAN may be provided. The transmission channel may bedivided into a plurality of subchannels allocated to the plurality ofSTAs. The executable instructions may cause the AP apparatus to generatea padding individually for one or more subchannels to which paddings areapplied among the plurality of subchannels, wherein the padding has alength such that transmissions end simultaneously on the plurality ofsubchannels, and to transmit a PPDU frame including a data unit withoutthe padding or a data unit added with the padding for each of theplurality of subchannels to the plurality of STAs on the transmissionchannel.

In another aspect of the present invention, a software orcomputer-readable medium having instructions executable for an STAapparatus to transmit data to an AP in a WLAN may be provided. Theexecutable instructions may cause the STA apparatus to receive a triggerframe from the AP, the trigger frame allocating a plurality ofsubchannels to the STA and one or more other STAs, to generate, when apadding is applied to a subchannel allocated to the STA, the paddinghaving a length such that a transmission on the subchannel allocated tothe STA and transmissions on one or more other subchannels allocated tothe one or more other STAs end simultaneously, and to transmit a PPDUframe including a data unit without the padding or a data unit addedwith the padding to the AP on the subchannel allocated to the STA.

It is to be understood that both the foregoing summarized features areexemplary aspects of the following detailed description of the presentinvention without limiting the scope of the present invention.

According to the present invention, a PPDU format including a padding ina HEW, and a method and apparatus for transmitting and receiving signalsusing the PPDU format can be provided.

The advantages of the present invention are not limited to the foregoingdescriptions, and additional advantages will become apparent to thosehaving ordinary skill in the pertinent art to the present inventionbased upon the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a block diagram of a Wireless Local Area Network (WLAN)device;

FIG. 2 is a schematic block diagram of an exemplary transmitting signalprocessing unit in a WLAN;

FIG. 3 is a schematic block diagram of an exemplary receiving signalprocessing unit in a WLAN;

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs);

FIG. 5 is a conceptual diagram illustrating a procedure for transmittinga frame in Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) for avoiding collisions between frames in a channel;

FIG. 6 depicts an exemplary frame structure in a WLAN system;

FIG. 7 depicts an exemplary High Efficiency (HE) Physical layer ProtocolData Unit (PPDU) frame format according to the present invention;

FIG. 8 depicts subchannel allocation in a HE PPDU frame format accordingto the present invention;

FIG. 9 depicts a subchannel allocation method according to the presentinvention;

FIG. 10 depicts the starting and ending points of an High EfficiencyLong Training Field (HE-LTF) field in a HE PPDU frame format accordingto the present invention;

FIG. 11 depicts a High Efficiency SIGnal B (HE-SIG-B) field and a HighEfficiency SIGnal C (HE-SIG-C) field in the HE PPDU frame formataccording to the present invention;

FIG. 12 depicts another exemplary HE PPDU frame format according to thepresent invention;

FIG. 13 depicts a configuration of an Aggregated-MAC Protocol Data Unit(A-MPDU);

FIG. 14 depicts an exemplary HE PPDU padding according to the presentinvention;

FIG. 15 depicts another exemplary HE PPDU padding according to thepresent invention;

FIG. 16 is a view comparing the lengths of MPDU fields on a plurality ofsubchannels in the example of FIG. 15;

FIG. 17 depicts another exemplary HE PPDU padding according to thepresent invention, and FIG. 18 illustrates an exemplary format of atrigger frame.

FIG. 19 is a flowchart illustrating an exemplary method according to thepresent invention; and

FIG. 20 is a flowchart illustrating another exemplary method accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, only certain embodiments of thepresent invention have been shown and described, simply by way ofillustration. As those skilled in the art would realize, the describedembodiments may be modified in various different ways, all withoutdeparting from the spirit or scope of the present invention.Accordingly, the drawings and description are to be regarded asillustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

In a Wireless Local Area network (WLAN), a Basic Service Set (BSS)includes a plurality of WLAN devices. A WLAN device may include a MediumAccess Control (MAC) layer and a PHYsical (PHY) layer according toInstitute of Electrical and Electronics Engineers (IEEE) 802.11 seriesstandards. In the plurality of WLAN devices, at least one the WLANdevice may be an Access Point (AP) and the other WLAN devices may benon-AP Stations (non-AP STAs). Alternatively, all of the plurality ofWLAN devices may be non-AP STAs in an ad-hoc networking environment. Ingeneral, AP STA and non-AP STA may be each referred to as a STA or maybe collectively referred to as STAs. However, for ease of descriptionherein, only the non-AP STAs may be referred to herein as the STAs.

FIG. 1 is a block diagram of a WLAN device.

Referring to FIG. 1, a WLAN device 1 includes a baseband processor 10, aRadio Frequency (RF) transceiver 20, an antenna unit 30, a memory 40, aninput interface unit 50, an output interface unit 60, and a bus 70.

The baseband processor 10 may be simply referred to as a processor,performs baseband signal processing described in the presentspecification, and includes a MAC processor (or MAC entity) 11 and a PHYprocessor (or PHY entity) 15.

In an embodiment of the present invention, the MAC processor 11 mayinclude a MAC software processing unit 12 and a MAC hardware processingunit 13. The memory 40 may store software (hereinafter referred to as‘MAC software’) including at least some functions of the MAC layer. TheMAC software processing unit 12 may execute the MAC software toimplement some functions of the MAC layer, and the MAC hardwareprocessing unit 13 may implement the remaining functions of the MAClayer in hardware (hereinafter referred to as ‘MAC hardware’). However,the MAC processor 11 is not limited to the foregoing implementationexamples.

The PHY processor 15 includes a transmitting (TX) signal processing unit100 and a receiving (RX) signal processing unit 200.

The baseband processor 10, the memory 40, the input interface unit 50,and the output interface unit 60 may communicate with one another viathe bus 70.

The RF transceiver 20 includes an RF transmitter 21 and an RF receiver22.

The memory 40 may further store an Operating System (OS) andapplications. The input interface unit 50 receives information from auser, and the output interface unit 60 outputs information to the user.

The antenna unit 30 includes one or more antennas. When Multiple inputMultiple Output (MIMO) or Multi-User MIMO (MU-MIMO) is used, the antennaunit 30 may include a plurality of antennas.

FIG. 2 is a schematic block diagram of an exemplary transmission signalprocessor in a WLAN.

Referring to FIG. 2, the transmitting signal processing unit 100 mayinclude an encoder 110, an interleaver 120, a mapper 130, an InverseFourier Transformer (IFT) 140, and a Guard Interval (GI) inserter 150.

The encoder 110 encodes input data. For example, the encoder 110 may bea Forward Error Correction (FEC) encoder. The FEC encoder may include aBinary Convolutional Code (BCC) encoder followed by a puncturing device,or the FEC encoder may include a Low-Density Parity-Check (LDPC)encoder.

The transmitting signal processing unit 100 may further include ascrambler for scrambling the input data before encoding to reduce theprobability of long sequences of 0s or 1s. If BCC encoding is used inthe encoder 110, the transmitting signal processing unit 100 may furtherinclude an encoder parser for demultiplexing the scrambled bits among aplurality of BCC encoders. If LDPC encoding is used in the encoder 110,the transmitting signal processing unit 100 may not use the encoderparser.

The interleaver 120 interleaves the bits of each stream output from theencoder 110 to change the order of bits. Interleaving may be appliedonly when BCC encoding is used in the encoder 110. The mapper 130 mapsthe sequence of bits output from the interleaver 120 to constellationpoints. If LDPC encoding is used in the encoder 110, the mapper 130 mayfurther perform LDPC tone mapping in addition to constellation mapping.

When MIMO or MU-MIMO is used, the transmitting signal processing unit100 may use a plurality of interleavers 120 and a plurality of mappers130 corresponding to the number of spatial streams, N_(SS). In thiscase, the transmitting signal processing unit 100 may further include astream parser for dividing outputs of the BCC encoders or output of theLDPC encoder into blocks that are sent to different interleavers 120 ormappers 130. The transmitting signal processing unit 100 may furtherinclude a Space-Time Block Code (STBC) encoder for spreading theconstellation points from the N_(SS) spatial streams into N_(STS)space-time streams and a spatial mapper for mapping the space-timestreams to transmit chains. The spatial mapper may use direct mapping,spatial expansion, or beamforming.

The IFT 140 converts a block of constellation points output from themapper 130 or the spatial mapper to a time-domain block (i.e., a symbol)by using Inverse Discrete Fourier Transform (IDFT) or Inverse FastFourier Transform (IFFT). If the STBC encoder and the spatial mapper areused, the IFT 140 may be provided for each transmit chain.

When MIMO or MU-MIMO is used, the transmitting signal processing unit100 may insert Cyclic Shift Diversities (CSDs) to prevent unintentionalbeamforming. The CSD insertion may occur before or after IFT. The CSDmay be specified per transmit chain or may be specified per space-timestream. Alternatively, the CSD may be applied as a part of the spatialmapper.

When MU-MIMO is used, some blocks before the spatial mapper may beprovided for each user.

The GI inserter 150 prepends a GI to the symbol. The transmitting signalprocessing unit 100 may optionally perform windowing to smooth edges ofeach symbol after inserting the GI. The RF transmitter 21 converts thesymbols into an RF signal and transmits the RF signal via the antennaunit 30. When MIMO or MU-MIMO is used, the GI inserter 150 and the RFtransmitter 21 may be provided for each transmit chain.

FIG. 3 is a schematic block diagram of an exemplary a receiving signalprocessor in a WLAN.

Referring to FIG. 3, the receiving signal processing unit 200 includes aGI remover 220, a Fourier Transformer (FT) 230, a demapper 240, adeinterleaver 250, and a decoder 260.

An RF receiver 22 receives an RF signal via the antenna unit 30 andconverts the RF signal into symbols. The GI remover 220 removes the GIfrom the symbol. When MIMO or MU-MIMO is used, the RF receiver 22 andthe GI remover 220 may be provided for each receive chain.

The FT 230 converts the symbol (i.e., the time-domain block) into ablock of constellation points by using a Discrete Fourier Transform(DFT) or a Fast Fourier Transform (FFT). The FT 230 may be provided foreach receive chain.

When MIMO or MU-MIMO is used, the receiving signal processing unit 200may include a spatial demapper for converting Fourier Transformedreceiver chains to constellation points of the space-time streams, andan STBC decoder for despreading the constellation points from thespace-time streams into the spatial streams.

The demapper 240 demaps the constellation points output from the FT 230or the STBC decoder to bit streams. If LDPC encoding is applied to thereceived signal, the demapper 240 may further perform LDPC tonedemapping before constellation demapping. The deinterleaver 250deinterleaves the bits of each stream output from the demapper 240.Deinterleaving may be applied only when a BCC encoding scheme is appliedto the received signal.

When MIMO or MU-MIMO is used, the receiving signal processing unit 200may use a plurality of demappers 240 and a plurality of deinterleavers250 corresponding to the number of spatial streams. In this case, thereceiving signal processing unit 200 may further include a streamdeparser for combining streams output from the deinterleavers 250.

The decoder 260 decodes the streams output from the deinterleaver 250 orthe stream deparser. For example, the decoder 100 may be an FEC decoder.The FEC decoder may include a BCC decoder or an LDPC decoder. Thereceiving signal processing unit 200 may further include a descramblerfor descrambling the decoded data. If BCC decoding is used in thedecoder 260, the receiving signal processing unit 200 may furtherinclude an encoder deparser for multiplexing the data decoded by aplurality of BCC decoders. If LDPC decoding is used in the decoder 260,the receiving signal processing unit 200 may not use the encoderdeparser.

In a WLAN system, Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) is a basic MAC access mechanism. The CSMA/CA mechanism isreferred to as Distributed Coordination Function (DCF) of IEEE 802.11MAC, shortly as a ‘listen before talk’ access mechanism. According tothe CSMA/CA mechanism, an AP and/or a STA may sense a medium or achannel for a predetermined time before starting transmission, that is,may perform Clear Channel Assessment (CCA). If the AP or the STAdetermines that the medium or channel is idle, it may start to transmita frame on the medium or channel. On the other hand, if the AP and/orthe STA determines that the medium or channel is occupied or busy, itmay set a delay period (e.g., a random backoff period), wait for thedelay period without starting transmission, and then attempt to transmita frame. By applying a random backoff period, a plurality of STAs areexpected to attempt frame transmission after waiting for different timeperiods, resulting in minimizing collisions.

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs).

WLAN devices may exchange data frames, control frames, and managementframes with each other.

A data frame is used for transmission of data forwarded to a higherlayer. The WLAN device transmits the data frame after performing backoffif a Distributed Coordination Function IFS (DIFS) has elapsed from atime when the medium has been idle. A management frame is used forexchanging management information which is not forwarded to the higherlayer. The WLAN device transmits the management frame after performingbackoff if an IFS such as the DIFS or a Point Coordination Function IFS(PIFS) has elapsed. Subtype frames of the management frame include abeacon frame, an association request/response frame, a proberequest/response frame, and an authentication request/response frame. Acontrol frame is used for controlling access to the medium. Subtypeframes of the control frame include a Request-To-Send (RTS) frame, aClear-To-Send (CTS) frame, and an ACKnowledgement (ACK) frame. In thecase that the control frame is not a response frame to another frame,the WLAN device transmits the control frame after performing backoff ifthe DIFS has elapsed. In case that the control frame is a response frameto another frame, the WLAN device transmits the control frame withoutperforming backoff if a Short IFS (SIFS) has elapsed. The type andsubtype of a frame may be identified by a type field and a subtype fieldin a Frame Control (FC) field.

On the other hand, a Quality of Service (QoS) STA transmits a frameafter performing backoff if an Arbitration IFS (AIFS) for an associatedAccess Category (AC), i.e., AIFS[i] (i is determined based on AC) haselapsed. In this case, the AIFC[i] may be used for a data frame, amanagement frame, or a control frame that is not a response frame.

In the example illustrated in FIG. 4, upon generation of a frame to betransmitted, a STA may transmit the frame immediately, if it determinesthat the medium is idle for the DIFS or AIFS[i] or longer. The medium isbusy for a time period during which the STA transmits the frame. Duringthe time period, upon generation of a frame to be transmitted, anotherSTA may defer access by confirming that the medium is busy. If themedium gets idle, the STA that intends to transmit the frame may performa backoff operation after a predetermined IFS in order to minimizecollision with any other STA. Specifically, the STA that intends totransmit the frame selects a random backoff count, waits for a slot timecorresponding to the selected random backoff count, and then attempttransmission. The random backoff count is determined based on aContention Window (CW) parameter and the medium is monitoredcontinuously during count-down of backoff slots (i.e. decrement abackoff count-down) according to the determined backoff count. If theSTA monitors the medium as busy, the STA discontinues the count-down andwaits, and then, if the medium gets idle, the STA resumes thecount-down. If the backoff slot count reaches 0, the STA may transmitthe next frame.

FIG. 5 is a conceptual diagram illustrating a CSMA/CA-based frametransmission procedure for avoiding collisions between frames in achannel.

Referring FIG. 5, a first STA (STA1) is a transmit WLAN device fortransmitting data, a second STA (STA2) is a receive WLAN device forreceiving the data from STA1, and a third STA (STA3) is a WLAN devicewhich may be located in an area where a frame transmitted from STA1and/or a frame transmitted from STA2 can be received by STA3.

STA1 may determine whether the channel is busy by carrier sensing. TheSTA1 may determine the channel occupation based on an energy level onthe channel or correlation of signals in the channel, or may determinethe channel occupation by using a Network Allocation Vector (NAV) timer.

After determining that the channel is not being used by other devicesduring DIFS (that is, the channel is idle), STA1 may transmit an RTSframe to STA2 after performing backoff. Upon receiving the RTS frame,STA2 may transmit a CTS frame as a response to the CTS frame after SIFS.

When STA3 receives the RTS frame, STA3 may set the NAV timer for atransmission duration of subsequently transmitted frame by usingduration information included in the RTS frame. For example, the NAVtimer may be set for a duration of SIFS+CTS frame duration+SIFS+dataframe duration+SIFS+ACK frame duration. When STA3 receives the CTSframe, it may set the NAV timer for a transmission duration ofsubsequently transmitted frames by using duration information includedin the CTS frame. For example, the NAV timer may be set for a durationof SIFS+a data frame duration+SIFS+an ACK frame duration. Upon receivinga new frame before the NAV timer expires, STA3 may update the NAV timerby using duration information included in the new frame. STA3 does notattempt to access the channel until the NAV timer expires.

When STA1 receives the CTS frame from STA2, it may transmit a data frameto STA2 after SIFS elapsed from the CTS frame has been completelyreceived. Upon successfully receiving the data frame, STA2 may transmitan ACK frame as a response to the data frame after SIFS elapsed.

When the NAV timer expires, STA3 may determine whether the channel isbusy through the use of carrier sensing. Upon determining that thechannel is not in use by other devices during DIFS and after the NAVtimer has expired, STA3 may attempt channel access after a contentionwindow after a random backoff has elapsed.

FIG. 6 depicts an exemplary frame structure in a WLAN system.

PHY layer may prepare a transmission MAC PDU (MPDU) in response to aninstruction (or a primitive, which is a set of instructions or a set ofparameters) by the MAC layer. For example, upon receipt of aninstruction requesting transmission start from the MAC layer, the PHYlayer may switch to a transmission mode, construct a frame withinformation (e.g., data) received from the MAC layer, and transmit theframe.

Upon detection of a valid preamble in a received frame, the PHY layermonitors a header of the preamble and transmits an instructionindicating reception start of the PHY layer to the MAC layer.

Information is transmitted and received in frames in the WLAN system.For this purpose, a Physical layer Protocol Data Unit (PPDU) frameformat is defined.

A PPDU frame may include a Short Training Field (STF) field, a LongTraining Field (LTF) field, a SIGNAL (SIG) field, and a Data field. Themost basic (e.g., a non-High Throughput (non-HT)) PPDU frame may includeonly a Legacy-STF (L-STF) field, a Legacy-LTF (L-LTF) field, a SIGfield, and a Data field. Additional (or other types of) STF, LTF, andSIG fields may be included between the SIG field and the Data fieldaccording to the type of a PPDU frame format (e.g., an HT-mixed formatPPDU, an HT-greenfield format PPDU, a Very High Throughput (VHT) PPDU,etc.).

The STF is used for signal detection, Automatic Gain Control (AGC),diversity selection, fine time synchronization, etc. The LTF field isused for channel estimation, frequency error estimation, etc. The STFand the LTF fields may be referred to as signals for OFDM PHY layersynchronization and channel estimation.

The SIG field may include a RATE field and a LENGTH field. The RATEfield may include information about a modulation scheme and coding rateof data. The LENGTH field may include information about the length ofthe data. The SIG field may further include parity bits, SIG TAIL bits,etc.

The Data field may include a SERVICE field, a Physical layer ServiceData Unit (PSDU), and PPDU TAIL bits. When needed, the Data field mayfurther include padding bits. A part of the bits of the SERVICE fieldmay be used for synchronization at a descrambler of a receiver. The PSDUcorresponds to a MAC PDU defined at the MAC layer and may include datagenerated/used in a higher layer. The PPDU TAIL bits may be used toreturn an encoder to a zero state. The padding bits may be used to matchthe length of the Data filed in predetermined units.

A MAC PDU is defined according to various MAC frame formats. A basic MACframe includes a MAC header, a frame body, and a Frame Check Sequence(FCS). The MAC frame includes a MAC PDU and may be transmitted andreceived in the PSDU of the data part in the PPDU frame format.

The MAC header includes a Frame Control field, a Duration/Identifier(ID) field, an Address field, etc. The Frame Control field may includecontrol information required for frame transmission/reception. TheDuration/ID field may be set to a time for transmitting the frame. Fordetails of Sequence Control, QoS Control, and HT Control subfields ofthe MAC header, refer to the IEEE 802.11-2012 technical specification.

The Frame Control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame, and Order subfields. For the contents ofeach subfield in the Frame Control field, refer to the IEEE 802.11-2012technical specification.

A Null-Data Packet (NDP) frame format is a frame format that does notinclude a data packet. In other words, the NDP frame format includesonly a Physical Layer Convergence Protocol (PLCP) header part (i.e., theSTF, LTF, and SIG fields) of the general PPDU frame format, without theremaining part (i.e., the Data field) of the general PPDU frame format.The NDP frame format may be referred to as a short frame format.

The IEEE 802.11ax task group is discussing a WLAN system, called a HighEfficiency WLAN (HEW) system, that operates in 2.4 GHz or 5 GHz andsupports a channel bandwidth (or channel width) of 20 MHz, 40 MHz, 80MHz, or 160 MHz. The present invention defines a new PPDU frame formatfor the IEEE 802.11ax HEW system. The new PPDU frame format may supportMU-MIMO or OFDMA. A PPDU of the new format may be referred to as a ‘HEWPPDU’ or ‘HE PPDU’ (similarly, HEW xyz may be referred to as ‘HE xyz’ or‘HE-xyz’ in the following descriptions).

In present specification, the term ‘MU-MIMO or OFDMA mode’ includesMU-MIMO without using OFDMA, or OFDMA mode without using MU-MIMO in anorthogonal frequency resource, or OFDMA mode using MU-MIMO in anorthogonal frequency resource.

FIG. 7 depicts an exemplary HE PPDU frame format according to thepresent invention.

Referring to FIG. 7, the vertical axis represents frequency and thehorizontal axis represents time. It is assumed that frequency and timeincrease in the upward direction and the right direction, respectively.

In the example of FIG. 7, one channel includes four subchannels. AnL-STF, an L-LTF, an L-SIG, and an HE-SIG-A may be transmitted perchannel (e.g., 20 MHz), a HE-STF and a HE-LTF may be transmitted on eachsubchannel being a basic subchannel unit (e.g., 5 MHz), and a HE-SIG-Band a PSDU may be transmitted on each of subchannels allocated to a STA.A subchannel allocated to a STA may have a size required for PSDUtransmission to the STA. The size of the subchannel allocated to the STAmay be N (N=1, 2, 3, . . . ) times as large as the size of basicsubchannel unit (i.e., a subchannel having a minimum size). In theexample of FIG. 7, the size of a subchannel allocated to each STA isequal to the size of the basic subchannel unit. For example, a firstsubchannel may be allocated for PSDU transmission from an AP to STA1 andSTA2, a second subchannel may be allocated for PSDU transmission fromthe AP to STA3 and STA4, a third subchannel may be allocated for PSDUtransmission from the AP to STA5, and a fourth subchannel may beallocated for PSDU transmission from the AP to STA6.

While the term subchannel is used in the present disclosure, the termsubchannel may be referred to as Resource Unit (RU) or subband. Inparticular, the terms like OFDMA subchannel, OFDMA RU, OFDMA subband canbe used in embodiments for OFDMA in the present disclosure. Terms like abandwidth of a subchannel, a number of tones (or subcarriers) allocatedto a subchannel, a number of data tones (or data subcarriers) allocatedto a subchannel can be used to express a size of a subchannel. Asubchannel refers to a frequency band allocated to a STA and a basicsubchannel unit refers to a basic unit used to represent the size of asubchannel. While the size of the basic subchannel unit is 5 MHz in theabove example, this is purely exemplary. Thus, the basic subchannel unitmay have a size of 2.5 MHz.

In FIG. 7, a plurality of HE-LTF elements are distinguished in the timeand frequency domains. One HE-LTF element may correspond to one OFDMsymbol in time domain and one subchannel unit (i.e., a subchannelbandwidth allocated to a STA) in frequency domain. The HE-LTF elementsare logical units and the PHY layer does not necessarily operate inunits of an HE-LTF element. In the following description, a HE-LTFelement may be referred to shortly as a HE-LTF.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in time domain and in one channel unit (e.g., 20 MHz) infrequency domain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in time domain and in one subchannel unit (i.e., asubchannel bandwidth allocated to a STA) in frequency domain.

A HE-LTF field may be a set of HE-LTF elements, HE-LTF symbols, orHE-LTF sections for a plurality of stations.

The L-STF field is used for frequency offset estimation and phase offsetestimation, for preamble decoding at a legacy STA (i.e., a STA operatingin a system such as IEEE 802.11a/b/g/n/ac). The L-LTF field is used forchannel estimation, for the preamble decoding at the legacy STA. TheL-SIG field is used for the preamble decoding at the legacy STA andprovides a protection function for PPDU transmission of a third-partySTA (e.g., setting a NAV based on the value of a LENGTH field includedin the L-SIG field).

HE-SIG-A (or HEW SIG-A) represents High Efficiency Signal A (or HighEfficiency WLAN Signal A), and includes HE PPDU (or HEW PPDU) modulationparameters, etc. for HE preamble (or HEW preamble) decoding at a HE STA(or HEW STA). The parameters set included in the HEW SIG-A field mayinclude one or more of Very High Throughput (VHT) PPDU modulationparameters transmitted by IEEE 802.11ac stations, as listed in [Table 1]below, to ensure backward compatibility with legacy STAs (e.g., IEEE802.11ac stations).

TABLE 1 Two parts of Number VHT-SIG-A Bit Field of bits DescriptionVHT-SIG-A1 B0-B1 BW 2 Set to 0 for 20 MHz, 1 for 40 MHz, 2 for 80 MHz,and 3 for 160 MHz and 80 + 80 MHz B2 Reserved 1 Reserved. Set to 1. B3STBC 1 For a VHT SU PPDU: Set to 1 if space time block coding is usedand set to 0 otherwise. For a VHT MU PPDU: Set to 0. B4-B9 Group ID 6Set to the value of the TXVECTOR parameter GROUP_ID. A value of 0 or 63indicates a VHT SU PPDU; otherwise, indicates a VHT MU PPDU. B10-B21NSTS/Partial 12 For a VHT MU PPDU: NSTS is divided into 4 user AIDpositions of 3 bits each. User position p, where 0 ≦ p ≦ 3, uses bitsB(10 + 3p) to B(12 + 3p). The number of space- time streams for user uare indicated at user position p = USER_POSITION[u] where u = 0, 1, . .. , NUM_USERS − 1 and the notation A[b] denotes the value of array A atindex b. Zero space-time streams are indicated at positions not listedin the USER_POSITION array. Each user position is set as follows: Set to0 for 0 space-time streams Set to 1 for 1 space-time stream Set to 2 for2 space-time streams Set to 3 for 3 space-time streams Set to 4 for 4space-time streams Values 5-7 are reserved For a VHT SU PPDU: B10-B12Set to 0 for 1 space-time stream Set to 1 for 2 space-time streams Setto 2 for 3 space-time streams Set to 3 for 4 space-time streams Set to 4for 5 space-time streams Set to 5 for 6 space-time streams Set to 6 for7 space-time streams Set to 7 for 8 space-time streams B13-B21 PartialAID: Set to the value of the TXVECTOR parameter PARTIAL_AID. Partial AIDprovides an abbreviated indication of the intended recipient(s) of thePSDU (see 9.17a). B22 TXOP_PS_(—) 1 Set to 0 by VHT AP if it allowsnon-AP VHT STAs in NOT_ALLOWED TXOP power save mode to enter Doze stateduring a TXOP. Set to 1 otherwise. The bit is reserved and set to 1 inVHT PPDUs transmitted by a non-AP VHT STA. B23 Reserved 1 Set to 1VHT-SIG-A2 B0 Short GI 1 Set to 0 if short guard interval is not used inthe Data field. Set to 1 if short guard interval is used in the Datafield. B1 Short GI 1 Set to 1 if short guard interval is used andN_(SYM) mod 10 = 9; N_(SYM) otherwise, set to 0. N_(SYM) is defined in22.4.3. Disambiguation B2 SU/MU[0] 1 For a VHT SU PPDU, B2 is set to 0for BCC, 1 for LDPC Coding For a VHT MU PPDU, if the MU[0] NSTS field isnonzero, then B2 indicates the coding used for user u withUSER_POSITION[u] = 0; set to 0 for BCC and 1 for LDPC. If the MU[0] NSTSfield is 0, then this field is reserved and set to 1. B3 LDPC Extra 1Set to 1 if the LDPC PPDU encoding process (if an SU OFDM PPDU), or atleast one LDPC user's PPDU encoding process Symbol (if a VHT MU PPDU),results in an extra OFDM symbol (or symbols) as described in 22.3.10.5.4and 22.3.10.5.5. Set to 0 otherwise. B4-B7 SU VHT-MCS/MU[1- 4 For a VHTSU PPDU: 3] Coding VHT-MCS index For a VHT MU PPDU: If the MU[1] NSTSfield is nonzero, then B4 indicates coding for user u withUSER_POSITION[u] = 1: set to 0 for BCC, 1 for LDPC. If the MU[1] NSTSfield is 0, then B4 is reserved and set to 1. If the MU[2] NSTS field isnonzero, then B5 indicates coding for user u with USER_POSITION[u] = 2:set to 0 for BCC, 1 for LDPC. If the MU[2] NSTS field is 0, then B5 isreserved and set to 1. If the MU[3] NSTS field is nonzero, then B6indicates coding for user u with USER_POSITlON[u] = 3: set to 0 for BCC,1 for LDPC. If the MU[3] NSTS field is 0, then B6 is reserved and setto 1. B7 is reserved and set to 1 B8 Beamformed 1 For a VHT SU PPDU: Setto 1 if a Beamforming steering matrix is applied to the waveform in anSU transmission as described in 20.3.11.11.2, set to 0 otherwise. For aVHT MU PPDU: Reserved and set to 1 NOTE-If equal to 1 smoothing is notrecommended. B9 Reserved 1 Reserved and set to 1 B10-B17 CRC 8 CRCcalculated as in 20.3.9.4.4 with c7 in B10. Bits 0-23 of HT-SIG1 andbits 0-9 of HT-SIG2 are replaced by bits 0-23 of VHT-SIG-A1 and bits 0-9of VHT-SIG-A2, respectively. B18-B23 Tail 6 Used to terminate thetrellis of the convolutional decoder. Set to 0.

[Table 1] illustrates fields, bit positions, numbers of bits, anddescriptions included in each of two parts, VHT-SIG-A1 and VHT-SIG-A2,of the VHT-SIG-A field defined by the IEEE 802.11ac standard. Forexample, a BW (BandWidth) field occupies two Least Significant Bits(LSBs), B0 and B1 of the VHT-SIG-A1 field and has a size of 2 bits. Ifthe 2 bits are set to 0, 1, 2, or 3, the BW field indicates 20 MHz, 40MHz, 80 MHz, or 160 and 80+80 MHz. For details of the fields included inthe VHT-SIG-A field, refer to the IEEE 802.11ac-2013 technicalspecification. In the HE PPDU frame format of the present invention, theHE-SIG-A field may include one or more of the fields included in theVHT-SIG-A field, and it may provide backward compatibility with IEEE802.11ac stations.

FIG. 8 depicts subchannel allocation in the HE PPDU frame formataccording to the present invention.

In the example of FIG. 8, it is assumed that information indicatingsubchannels to which STAs are allocated in HE PPDU indicates that asubchannel of 0 MHz is allocated to STA1 (i.e., no subchannel isallocated), a subchannel of 5 MHz is allocated to each of STA2 and STA3,and a subchannel of 10 MHz is allocated to STA4.

In the example of FIG. 8, an L-STF, an L-LTF, an L-SIG, and a HE-SIG-Amay be transmitted per channel (e.g., 20 MHz), a HE-STF and a HE-LTF maybe transmitted on each of subchannels being basic subchannel units(e.g., 5 MHz), and a HE-SIG-B and a PSDU may be transmitted on each ofsubchannels allocated to STAs. A subchannel allocated to a STA has asize required for PSDU transmission to the STA. The size of thesubchannel allocated to the STA may be an N (N=1, 2, 3, . . . ) multipleof the size of the basic subchannel unit (i.e., a minimum-sizesubchannel unit). In the example of FIG. 8, the size of a subchannelallocated to STA2 is equal to that of the basic subchannel unit, thesize of a subchannel allocated to STA3 is equal to that of the basicsubchannel unit, and the size of a subchannel allocated to STA4 is twicelarger than that of the basic subchannel unit.

FIG. 8 illustrates a plurality of HE-LTF elements and a plurality ofHE-LTF subelements which are distinguished in the time and frequencydomains. One HE-LTF element may correspond to one OFDM symbol in thetime domain and one subchannel unit (i.e., the bandwidth of a subchannelallocated to a STA) in the frequency domain. One HE-LTF subelement maycorrespond to one OFDM symbol in the time domain and one basicsubchannel unit (e.g. 5 MHz) in the frequency domain. In the example ofFIG. 8, one HE-LTF element includes one HE-LTF subelement in the 5-MHzsubchannel allocated to STA2 or STA3. On the other hand, one HE-LTFelement includes two HE-LTF subelements in the third subchannel, i.e.,10-MHz subchannel, allocated to STA4. A HE-LTF element and a HE-LTFsubelement are logical units and the PHY layer does not always operatein units of a HE-LTF element or HE-LTF subelement.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in the time domain and one channel unit (e.g. 20 MHz) in thefrequency domain. That is, one HE-LTF symbol may be divided into HE-LTFelements by a subchannel width allocated to a STA and into HE-LTFsubelements by the width of the basic subchannel unit in the frequencydomain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one subchannel unit (i.e. thebandwidth of a subchannel allocated to a STA) in the frequency domain. AHE-LTF subsection may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one basic subchannel unit(e.g., 5 MHz) in the frequency domain. In the example of FIG. 8, oneHE-LTF section includes one HE-LTF subsection in the 5-MHz subchannelallocated to STA2 or STA3. On the other hand, one HE-LTF sectionincludes two HE-LTF subsections in the third subchannel, i.e., 10-MHzsubchannel, allocated to STA4.

A HE-LTF field may correspond to a set of HE-LTF elements (orsubelements), HE-LTF symbols, or HE-LTF sections (or subsections) for aplurality of stations.

For the afore-described HE PPDU transmission, subchannels allocated to aplurality of HE STAs may be contiguous in the frequency domain. In otherwords, for HE PPDU transmission, the subchannels allocated to the HESTAs may be sequential and any intermediate one of the subchannels ofone channel (e.g., 20 MHz) may not be allowed to be unallocated orempty. Referring to FIG. 7, if one channel includes four subchannels, itmay not be allowed to keep the third subchannel unallocated and empty,while the first, second, and fourth subchannels are allocated to STAs.However, the present invention does not exclude non-allocation of aintermediate subchannel of one channel to a STA.

FIG. 9 depicts a subchannel allocation method according to the presentinvention.

In the example of FIG. 9, a plurality of contiguous channels (e.g.,20-MHz-bandwidth channels) and boundaries of the plurality of contiguouschannels are shown. In FIG. 9, a preamble may correspond to an L-STF, anL-LTF, an L-SIG, and a HE-SIG-A as illustrated in the examples of FIGS.7 and 8.

A subchannel for each HE STA may be allocated only within one channel,and may not be allocated with partially overlapping between a pluralityof channels. That is, if there are two contiguous 20-MHz channels CH1and CH2, subchannels for STAs paired for MU-MIMO-mode or OFDMA-modetransmission may be allocated either within CH1 or within CH2, and itmay be prohibited that one part of a subchannel exists in CH1 andanother part of the subchannel exists in CH2. This means that onesubchannel may not be allocated with crossing a channel boundary. Fromthe perspective of RUs supporting the MU-MIMO or OFDMA mode, a bandwidthof 20 MHz may be divided into one or more RUs, and a bandwidth of 40 MHzmay be divided into one or more RUs in each of two contiguous 20-MHzbandwidths, and no RU is allocated with crossing the boundary betweentwo contiguous 20-MHz bandwidths.

As described above, it is not allowed that one subchannel belongs to twoor more 20-MHz channels. Particularly, a 2.4-GHz OFDMA mode may supporta 20-MHz OFDMA mode and a 40-MHz OFDMA mode. In the 2.4-GHz OFDMA mode,it may not be allowed that one subchannel belongs to two or more 20-MHzchannels.

FIG. 9 is based on the assumption that subchannels each having the sizeof a basic subchannel unit (e.g., 5 MHz) in CH1 and CH2 are allocated toSTA1 to STA7, and subchannels each having double the size (e.g., 10 MHz)of the basic subchannel unit in CH4 and CH5 are allocated to STA8, STA9,and STA10.

As illustrated in the lower part of FIG. 9, although a subchannelallocated to STA1, STA2, STA3, STA5, STA6, or STA7 is fully overlappedonly with one channel (i.e., without crossing the channel boundary, orbelonging only to one channel), a subchannel allocated to STA4 ispartially overlapped with the two channels (i.e., crossing the channelboundary, or belonging to the two channels). In the forgoing example ofthe present invention, the subchannel allocation to STA4 is not allowed.

As illustrated in the upper part of FIG. 9, although a subchannelallocated to STA8 or STA10 is fully overlapped only with one channel(i.e., crossing the channel boundary, or belonging only to one channel),a subchannel allocated to STA9 is partially overlapped with two channels(i.e., crossing the channel boundary, or belonging to the two channels).In the forgoing example of the present invention, the subchannelallocation to STA9 is not allowed.

On the other hand, it may be allowed to allocate a subchannel partiallyoverlapped between a plurality of channels (i.e., crossing the channelboundary, or belonging to two channels). For example, in SU-MIMO modetransmission, a plurality of contiguous channels may be allocated to aSTA and any of one or more subchannels allocated to the STA may crossthe boundary between two contiguous channels.

While the following description is given with an assumption that onesubchannel has a channel bandwidth of 5 MHz in one channel having achannel bandwidth of 20 MHz, this is provided to simplify thedescription of the principle of the present invention and thus shouldnot be construed as limiting the present invention. For example, thebandwidths of a channel and a subchannel may be defined or allocated asvalues other than the above examples. In addition, a plurality ofsubchannels in one channel may have the same or different channelwidths.

FIG. 10 depicts the starting and ending points of a HE-LTF field in theHE PPDU frame format according to the present invention.

To support the MU-MIMO mode and the OFDMA mode, the HE PPDU frame formataccording to the present invention may include, in the HE-SIG-A field,information about the number of spatial streams to be transmitted to aHE STA allocated to each subchannel.

If MU-MIMO-mode or OFDMA-mode transmission is performed to a pluralityof HE STAs on one subchannel, the number of spatial streams to betransmitted to each of the HE STAs may be provided in the HE-SIG-A orHE-SIG-B field, which will be described later in detail.

FIG. 10 is based on the assumption that a first 5-MHz subchannel isallocated to STA1 and STA2 and two spatial streams are transmitted toeach STA in a DL MU-MIMO or OFDMA mode (i.e., a total of four spatialstreams are transmitted on one subchannel). For this purpose, a HE-STF,a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF, and a HE-SIG-B follow theHE-SIG-A field on the subchannel. The HE-STF is used for frequencyoffset estimation and phase offset estimation for the 5-MHz subchannel.The HE-LTFs are used for channel estimation for the 5-MHz subchannel.Since the subchannel carries four spatial streams, as many HE-LTFs(i.e., HE-LTF symbols or HE-LTF elements in a HE-LTF section) as thenumber of the spatial streams, that is, four HE-LTFs are required tosupport MU-MIMO transmission.

According to an example of the present invention, a relationship betweena number of total spatial streams transmitted in one subchannel and anumber of HE-LTF are listed in [Table 2].

TABLE 2 Total number of spatial streams Number of transmitted on onesubchannel HE-LTFs 1 1 2 2 3 4 4 4 5 6 6 6 7 8 8 8

Referring to [Table 2], if one spatial stream is transmitted on onesubchannel, at least one HE-LTF needs to be transmitted on thesubchannel. If an even number of spatial streams are transmitted on onesubchannel, at least as many HE-LTFs as the number of the spatialstreams need to be transmitted. If an odd number of spatial streamsgreater than one are transmitted on one subchannel, at least as manyHE-LTFs as a number of adding 1 to the number of the spatial streamsneed to be transmitted.

Referring to FIG. 10 again, it is assumed that the second 5-MHzsubchannel is allocated to STA3 and STA4 and one spatial streams per STAis transmitted in the DL MU-MIMO or OFDMA mode (i.e., a total of twospatial streams are transmitted on one subchannel). In this case, twoHE-LTFs need to be transmitted on the second subchannel, however, in theexample of FIG. 10, a HE-STF, a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF,and a HE-SIG-B follow the HE-SIG-A field on the subchannel (i.e., fourHE-LTFs are transmitted). This is for setting the same starting time ofPSDU transmission for subchannels allocated to other STAs paired withSTA3 and STA4 for MU-MIMO transmission. If only two HE-LTFs aretransmitted on the second subchannel, PSDUs are transmitted at differenttime points on the first and second subchannels. PSDU transmission oneach subchannel at a different time point results in discrepancy betweenOFDM symbol timings of subchannels, thereby no orthogonality ismaintained. To overcome this problem, an additional constraint need tobe imposed for HE-LTF transmission.

Basically, transmission of as many HE-LTFs as required is sufficient inan SU-MIMO or non-OFDMA mode. However, timing synchronization (oralignment) with fields transmitted on subchannels for other paired STAsis required in the MU-MIMO or OFDMA mode. Accordingly, the numbers ofHE-LTFs may be determined for all other subchannels based on asubchannel having the maximum number of streams in MU-MIMO-mode orOFDMA-mode transmission.

Specifically, the numbers of HE-LTFs may be determined for allsubchannels according to the maximum of the numbers of HE-LTFs (HE-LTFsymbols or HE-LTF elements in a HE-LTF section) required according tothe total numbers of spatial streams transmitted on each subchannel, fora set of HE STAs allocated to each subchannel. A “set of HE STAsallocated to each subchannel” is one HE STA in the SU-MIMO mode, and aset of HE STAs paired across a plurality of subchannels in the MU-MIMOmode. The ‘number of spatial streams transmitted on each subchannel’ isthe number of spatial streams transmitted to one HE STA in the SU-MIMOmode, and the number of spatial streams transmitted to a plurality of HESTAs paired on the subchannel in the MU-MIMO mode.

That is, it may be said that a HE-LTF field starts at the same timepoint and ends at the same time point in a HE PPDU for all users (i.e.HE STAs) in MU-MIMO-mode or OFDMA-mode transmission. Or it may be saidthat the lengths of HE-LTF sections are equal on a plurality ofsubchannels for all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-modetransmission. Or it may be said that the number of HE-LTF elementsincluded in each HE-LTF section is equal on a plurality of subchannelsfor all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-mode transmission.Accordingly, PSDU transmission timings may be synchronized among aplurality of subchannels for all HE STAs in MU-MIMO-mode or OFDMA-modetransmission.

As described above, the number of HE-LTF symbols (refer to FIG. 7) maybe 1, 2, 4, 6, or 8 in HE PPDU transmission in the MU-MIMO or OFDMAmode, determined according to the maximum of the numbers of spatialstreams on each of a plurality of subchannels. A different number ofspatial streams may be allocated to each of a plurality of subchannels,and the number of spatial streams allocated to one subchannel is thenumber of total spatial streams for all users allocated to thesubchannel. That is, the number of HE-LTF symbols may be determinedaccording to the number of spatial streams allocated to a subchannelhaving a maximum number of spatial streams by comparing the number oftotal spatial streams for all users allocated to one of a plurality ofsubchannels with the number of total spatial streams for all usersallocated to another subchannel.

Specifically, in HE PPDU transmission in the OFDMA mode, the number ofHE-LTF symbols may be 1, 2, 4, 6, or 8, determined based on the numberof spatial streams transmitted in a subchannel having a maximum numberof spatial streams across a plurality of subchannels. Further, in HEPPDU transmission in the OFDMA mode, the number of HE-LTF symbols may bedetermined based on whether the number of spatial streams transmitted ina subchannel having a maximum number of spatial streams across aplurality of subchannels is odd or even (refer to [Table 2]). That is,in HE PPDU transmission in the OFDMA mode, when the number (e.g., K) ofspatial streams transmitted in a subchannel having a maximum number ofspatial streams across a plurality of subchannels is an even number, thenumber of HE-LTF symbols may be equal to K. In HE PPDU transmission inthe OFDMA mode, when the number, K, of spatial streams transmitted in asubchannel having a maximum number of spatial streams across a pluralityof subchannels is an odd number greater than one, the number of HE-LTFsymbols may be equal to K+1.

When only one STA is allocated to one subchannel in OFDMA mode (i.e.,OFDMA mode without using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of spatial streams for a STA allocated to each subchannel.When more than one STA is allocated to one subchannel in OFDMA mode(i.e., OFDMA mode using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of STAs allocated to each subchannel and the number ofspatial streams for each STA allocated to each subchannel (e.g., if STA1and STA2 are allocated to one subchannel, sum of the number of spatialstreams for STA1 and the number of spatial streams for STA2).

When transmitting a HE PPDU frame in the MU-MIMO or OFDMA mode, atransmitter may generate P (P is an integer equal to or larger than 1)HE-LTF symbols (refer to FIG. 7) and transmit a HE PPDU frame includingat least the P HE-LTF symbols and a Data field to a receiver. The HEPPDU frame may be divided into Q subchannels in the frequency domain (Qis an integer equal to or larger than 2). Each of the P HE-LTF symbolsmay be divided into Q HE-LTF elements corresponding to the Q subchannelsin the frequency domain. That is, the HE PPDU may include P HE-LTFelements on one subchannel (herein, the P HE-LTF elements may belong toone HE-LTF section on the subchannel).

As described above, the number of HE-LTF elements (i.e., P) in one ofthe Q subchannels may be equal to the number of HE-LTF elements (i.e. P)of another subchannel. Also, the number of HE-LTF elements (i.e., P)included in a HE-LTF section in one of the Q subchannels may be equal tothe number of HE-LTF elements (i.e. P) included in a HE-LTF section inanother subchannel. The HE-LTF section of one of the Q subchannels maystart and end at the same time points as the HE-LTF section of anothersubchannel. Also, the HE-LTF sections may start and end at the same timepoints across the Q subchannels (i.e., across all users or stations).

Referring to FIG. 10 again, the third 5-MHz subchannel is allocated toSTA5 and one spatial stream is transmitted on the subchannel in SU-MIMO(considering all subchannels, a plurality of spatial streams aretransmitted to STA1 to STA6 in MU-MIMO or OFDMA mode). In this case,although transmission of one HE-LTF is sufficient for the subchannel, asmany HE-LTFs as the maximum of the numbers of HE-LTFs on the othersubchannels, that is, four HE-LTFs are transmitted on the subchannel inorder to align the starting points and ending points of the HE-LTFfields of the subchannels.

The fourth 5-MHz subchannel is allocated to STA6 and one spatial streamis transmitted on the subchannel in SU-MIMO (considering all othersubchannels, a plurality of spatial streams are transmitted to STA1 toSTA6 in MU-MIMO or OFDMA mode). In this case, although transmission ofone HE-LTF is sufficient for the subchannel, as many HE-LTFs as themaximum of the numbers of HE-LTFs on the other subchannels, that is,four HE-LTFs are transmitted on the subchannel in order to align thestarting points and ending points of the HE-LTF fields of thesubchannels.

In the example of FIG. 10, the remaining two HE-LTFs except two HE-LTFsrequired for channel estimation of STA3 and STA4 on the secondsubchannel, the remaining three HE-LTFs except one HE-LTF required forchannel estimation of STA5 on the third subchannel, and the remainingthree HE-LTFs except one HE-LTF required for channel estimation of STA6on the fourth subchannel may be said to be placeholders that areactually not used for channel estimation at the STAs.

FIG. 11 depicts a HE-SIG-B field and a HE-SIG-C field in the HE PPDUframe format according to the present invention.

To effectively support MU-MIMO-mode or OFDMA-mode transmission in the HEPPDU frame format according to the present invention, independentsignaling information may be transmitted on each subchannel.Specifically, a different number of spatial streams may be transmittedto each of a plurality of HE STAs that receive an MU-MIMO-mode orOFDMA-mode transmission simultaneously. Therefore, information about thenumber of spatial streams to be transmitted should be indicated to eachHE STA.

Information about the number of spatial streams on one channel may beincluded in, for example, a HE-SIG-A field. A HE-SIG-B field may includespatial stream allocation information about one subchannel. Also, aHE-SIG-C field may be transmitted after transmission of HE-LTFs,including MCS information about a PSDU and information about the lengthof the PSDU, etc.

With reference to the foregoing examples of the present invention,mainly the features of a HE PPDU frame structure applicable to a DLMU-MIMO-mode or OFDMA-mode transmission that an AP transmitssimultaneously to a plurality of STAs have been described. Now, adescription will be given of the features of a HE PPDU frame structureapplicable to a UL MU-MIMO-mode or OFDMA-mode transmission that aplurality of STAs transmits simultaneously to an AP.

The above-described various examples of structures of the HE PPDU frameformat supporting MU-MIMO-mode or OFDMA-mode transmission are notapplicable only to DL but also applicable UL. For example, theabove-described exemplary HE PPDU frame formats may also be used for aUL HE PPDU transmission that a plurality of STAs simultaneouslytransmits to a single AP.

However, in the case of a DL MU-MIMO-mode or OFDMA-mode HE PPDUtransmission that an AP simultaneously transmits to a plurality of STAs,the transmission entity, AP has knowledge of the number of spatialstreams transmitted to a HE STA allocated to each of a plurality ofsubchannels. Therefore, the AP may include, in a HE-SIG-A field or aHE-SIG-B field, information about the total number of spatial streamstransmitted across a channel, a maximum number of spatial streams (i.e.,information being a basis of the number of HE-LTF elements (or thestarting point and ending point of a HE-LTF section) on eachsubchannel), and the number of spatial streams transmitted on eachsubchannel. In contrast, in the case of a UL MU-MIMO-mode or OFDMA-modeHE PPDU transmission that a plurality of STAs simultaneously transmitsto an AP, each STA being a transmission entity may be aware only of thenumber of spatial streams in a HE PSDU that it will transmit, withoutknowledge of the number of spatial streams in a HE PSDU transmitted byanother STA paired with the STA. Accordingly, the STA may determineneither the total number of spatial streams transmitted across a channelnor a maximum number of spatial streams.

To solve this problem, a common parameter (i.e., a parameter appliedcommonly to STAs) and an individual parameter (a separate parameterapplied to an individual STA) may be configured as follows in relationto a UL HE PPDU transmission.

For simultaneous UL HE PPDU transmissions from a plurality of STAs to anAP, a protocol may be designed in such a manner that the AP sets acommon parameter or individual parameters (common/individual parameters)for the STAs for the UL HE PPDU transmissions and each STA operatesaccording to the common/individual parameters. For example, the AP maytransmit a trigger frame (or polling frame) for a UL MU-MIMO-mode orOFDMA-mode transmission to a plurality of STAs. The trigger frame mayinclude a common parameter (e.g., the number of spatial streams across achannel or a maximum number of spatial streams) and individualparameters (e.g., the number of spatial streams allocated to eachsubchannel), for the UL MU-MIMO-mode or OFDMA-mode transmission. As aconsequence, a HE PPDU frame format applicable to a UL MU-MIMO or OFDMAmode may be configured without a modification to an exemplary HE PPDUframe format applied to a DL MU-MIMO or OFDMA mode. For example, eachSTA may configure a HE PPDU frame format by including information aboutthe number of spatial streams across a channel in a HE-SIG-A field,determining the number of HE-LTF elements (or the starting point andending point of a HE-LTE section) on each subchannel according to themaximum number of spatial streams, and including information about thenumber of spatial streams for the individual STA in a HE-SIG-B field.

Alternatively, if the STAs operate always according to thecommon/individual parameters received in the trigger frame from the AP,each STA does not need to indicate the common/individual parameters tothe AP during a HE PPDU transmission. Therefore, this information maynot be included in a HE PPDU. For example, each STA may have only todetermine the total number of spatial streams, the maximum number ofspatial streams, and the number of spatial streams allocated toindividual STA, as indicated by the AP, and configure a HE PPDUaccording to the determined numbers, without including information aboutthe total number of spatial streams or the number of spatial streamsallocated to the STA in the HE PPDU.

On the other hand, if the AP does not provide common/individualparameters in a trigger frame, for a UL MIMO-mode or OFDMA-mode HE PPDUtransmission, the following operation may be performed.

Common transmission parameters (e.g., channel BandWidth (BW)information, etc.) for simultaneously transmitted HE PSDUs may beincluded in HE-SIG-A field, but parameters that may be different forindividual STAs (e.g., the number of spatial streams, an MCS, andwhether STBC is used or not, for each individual STA) may not beincluded in HE-SIG-A field. Although the individual parameters may beincluded in HE-SIG-B field, information about the number of spatialstreams and information indicating whether STBC is used or not, need tobe transmitted before a HE-LTF field because the number of spatialstreams and the information indicating whether STBC is used or not aresignificant to determination of configuration information about apreamble and a PSDU in a HE PPDU frame format (e.g., the number ofHE-LTF elements is determined according to a combination of the numberof spatial streams and the information indicating whether STBC is usedor not). For this purpose, a HE PPDU frame format as illustrated in FIG.12 may be used for a UL HE PPDU transmission.

FIG. 12 depicts another exemplary HE PPDU frame format according to thepresent invention. The HE PPDU frame format illustrated in FIG. 12 ischaracterized in that a structure of HE-SIG-A, HE-SIG-B, and HE-SIG-Cfields similar to in FIG. 10 is used for a UL PPDU transmission.

As described before, if a UL MU-MIMO-mode or OFDMA-mode transmission isperformed by triggering of an AP (according to common/individualparameters provided by the AP), an individual STA may not need to reportan individual parameter to the AP. In this case, one or more of aHE-SIG-B field, a HE-SIG-C field, and a first HE-LTF element (i.e., aHE-LTF between a HE-STF field and a HE-SIG-B field) illustrated in FIG.12 may not exist. In this case, a description of each field given belowmay be applied only in the presence of the field.

In the example of FIG. 12, a HE-SIG-A field is transmitted per channel(i.e., per 20-MHz channel) and may include transmission parameterscommon to simultaneously transmitted HE PSDUs. Since the sameinformation is transmitted in up to HE-SIG-A fields in UL PPDUstransmitted by HE STAs allocated to subchannels, the AP may receive thesame signals from the plurality of STAs successfully.

A HE-SIG-B field is transmitted per subchannel in one channel. TheHE-SIG-B field may have an independent parameter value according to thetransmission characteristics of a HE PSDU transmitted on eachsubchannel. The HE-SIG-B field may include spatial stream allocationinformation and information indicating whether STBC is used or not, foreach subchannel. If MU-MIMO is applied to a subchannel (i.e., if aplurality of STAs perform transmission on a subchannel), the HE-SIG-Bfield may include a common parameter for the plurality of STAs paired onthe subchannel.

A HE-SIG-C field is transmitted on the same subchannel as the HE-SIG-Bfield and may include information about an MCS and a packet length. IfMU-MIMO is applied to a subchannel (i.e., if a plurality of STAs performtransmission on a subchannel), the HE-SIG-C field may include respectiveindividual parameters for each of the plurality of STAs paired on thesubchannel.

Similarly to DL MU-MIMO-mode or OFDMA-mode HE PPDU transmission,transmissions of PSDUs may start at different time points on subchannelsin UL MU-MIMO-mode or OFDMA-mode HE PPDU transmission, and if OFDMsymbols are not aligned accordingly, then the implementation complexityof an AP that receives a plurality of PSDUs increased. To solve thisproblem, ‘the number of HE-LTFs may be determined for all subchannelsaccording to the maximum of the numbers of HE LTFs required according tothe total numbers of spatial streams transmitted on each subchannel fora set of HE STAs allocated to each of subchannels’ as described withreference to the example of FIG. 10.

This feature may mean that the HE-LTF field start at the same time pointand end at the same time point across all users (i.e., HE STAs) in ULMU-MIMO-mode or OFDMA-mode transmission. Or it may be said that theHE-LTF sections of a plurality of subchannels have the same lengthacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission. Or itmay be said that each of the HE-LTF sections of a plurality ofsubchannels includes the same number of HE-LTF elements across all HESTAs in UL MU-MIMO-mode or OFDMA-mode transmission. Therefore, PSDUtransmission timings are synchronized between a plurality of subchannelsacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission.

As described before, a plurality of STAs may simultaneously transmitPSDUs in a HE PPDU frame format to an AP on subchannels allocated to theSTAs (i.e., referred to as UL MU-MIMO or OFDMA transmission or “UL MUtransmission”), and a plurality of STAs may simultaneously receive aPSDU in a HE PPDU frame format from an AP on subchannels allocated tothe STAs (i.e., referred to as DL MU-MIMO or OFDMA transmission or “DLMU transmission”).

A padding applied to a HE PPDU for DL/UL MU transmission according tothe present invention will be described below.

Although a DL/UL MU PPDU supports simultaneous transmission to/from aplurality of STAs, data transmitted to/from the STAs may have differentlengths. If different subchannels (or resource units) are allocated tothe plurality of STAs and the STAs are allowed to terminate DL/ULtransmissions at different time points on the different subchannels,another device may access a subchannel on which transmission has beenterminated early, thereby making it impossible to protect DL/UL MUtransmission.

Further, an STA/AP that receives DL/UL data in a DL/UL MU PPDU mayprocess (e.g., decode) received data within a predetermined time (e.g.,an SIFS) after the data reception and transmit an ACK for the receiveddata. Since the DL/UL MU PPDU may include a large amount of data in aframe, the STA/AP may have difficulty in generating and transmitting anACK within the predetermined time (e.g., the SIFS). Accordingly, theDL/UL MU transmission should be terminated simultaneously on thesubchannels allocated to the plurality of STAs in the DL/UL MU PPDU. Forthis purpose, a padding may be applied to the DL/UL MU PPDU in thepresent invention. Because the padding corresponds to a non-datatransmission time period (i.e., a time period over which no actual datato be received is transmitted to a receiver of the DL/UL MU PPDU), thepadding may be applied for the purpose of securing a time for processingdata in the STA/AP receiving the DL/UL MU PPDU. A detailed descriptionwill be given of specific embodiments of a padding applied to a DL/UL MUPPDU according to the present invention.

First, a plurality of types of paddings according to the presentinvention will be described in detail with reference to FIG. 13. In thepresent invention, a plurality of types of paddings may be defined andused. The plurality of types of paddings include MAC padding, PHYpadding, and extension padding and two or more of the padding types maybe used.

FIG. 13 depicts a configuration of an Aggregated-MAC Protocol Data Unit(A-MPDU).

To transmit a plurality of MPDUs in one PPDU frame, the MPDUs may beaggregated. As illustrated in FIG. 13, the MAC layer may configure anA-MPDU by logically concatenating a plurality of MPDUs. The A-MPDU mayinclude a plurality of A-MPDU subframes. An A-MPDU subframe may includean MPDU Delimiter and an MPDU, and when needed, the A-MPDU may furtherinclude a PAD. One MPDU may be configured by including one MSDU orA-MSDU (i.e., configured by concatenating a plurality of MSDUs) andattaching an MPDU Header and an FCS (i.e., CRC) before and after theMSDU/A-MSDU, respectively.

According to the present invention, MAC padding includes adding anecessary number of padding bits after an MPDU including actual payload.For example, an MPDU (i.e., an A-MPDU subframe) of a predetermined sizemay be added after an MPDU including actual payload. If one MPDUincluding actual payload is transmitted, an A-MPDU subframe of apredetermined size may be added after an A-MPDU subframe including theMPDU. If a plurality of MPDUs including actual payload are transmitted,an A-MPDU subframe of a predetermined size may be added after an A-MPDUsubframe including the last of the plurality of MPDUs.

The MPDU (or A-MPDU subframe) of the predetermined size added for MACpadding may correspond to one or more 4-octect MPDUs (or A-MPDUsubframes) having null data (e.g., an MSDU of 0 octet). That is, theMPDU (or A-MPDU subframe) of the predetermined size added for MACpadding may correspond to one or more 4-octet MPDUs (or A-MPDUsubframes) each having an MPDU Length field set to 0 (i.e., a null MPDU)and an End Of Frame (EOF) set to 1 (i.e., EOF of an A-MPDU) in the MPDUDelimiter.

Further, a MAC padding may be applied individually to each subchannel(or resource unit) of a DL/UL MU PPDU according to the presentinvention. For example, a plurality of subchannels allocated within onetransmission channel (e.g., a 20-MHz bandwidth) may be allocated to aplurality of STAs. Although a MAC padding may be applied to a firstsubchannel of the DL/UL MU PPDU, a MAC padding may not be applied (i.e.,a MAC padding of size 0 may be applied) to a second subchannel of theDL/UL MU PPDU. Further, even though a MAC padding is applied to each ofthe plurality of subchannels in the DL/UL MU PPDU, the size of MACpadding bits of the first subchannel may be different from the size ofMAC padding bits of the second subchannel. The lengths of the MACpaddings applied to the individual subchannels may be determined so thatthe DATA fields of the plurality of subchannels may end at the same timepoint in the DL/UL MU PPDU.

In addition to MAC paddings, PHY paddings may be applied to the DL/UL MUPPDU.

The PHY layer may configure an A-MPDU with one PSDU. A PPDU frameincludes a preamble (e.g., a legacy preamble (i.e., an L-STF, an L-LTF,and an L-SIG) and a HE-preamble (i.e., a HE-SIG-A, a HE-LTF, etc.), andone or more PSDUs. When needed, the PPDU frame may further include a PADfield (i.e., PHY padding bits). The PHY padding is used to match thenumber of coded bits of the last OFDM symbol to a predeterminedcriterion (e.g., an integer multiple of a parameter value (i.e., thevalue of an N_(CBPS) parameter) for coded bits per OFDM symbol). The PHYpadding may be applied to each individual subchannel (or resource unit)of the DL/UL MU PPDU. Further, a PHY padding of the same value may beapplied commonly to the subchannels (or resource units) of the DL/UL MUPPDU.

In addition to a MAC padding and a PHY padding, an extension padding maybe added to the end of the DL/UL MU PPDU.

The extension padding may be added after the PPDU is configured with theMAC padding and the PHY padding and may correspond to a time length thatextends signal emission after the last symbol of the PPDU. For example,if the DL/UL MU PPDU is transmitted in 2.4 GHz, the extension paddingmay correspond to a Signal Extension field or packet extension having atime length of 6 μs, which should not be construed as limiting thepresent invention. The extension padding may be applied to eachindividual subchannel (or resource unit) of the DL/UL MU PPDU. Further,an extension padding of the same value may be applied commonly to thesubchannels (or resource units) of the DL/UL MU PPDU.

FIG. 14 depicts an exemplary HE PPDU padding according to the presentinvention.

The example of FIG. 14 corresponds to a case in which the transmissionstart timings of PSDUs are identical (or aligned) on a plurality ofsubchannels in a HE PPDU format. If HE-LTF fields start at the same timepoint and end at the same time point on the plurality of subchannels(i.e., the lengths of the HE-LTF fields are equal across the pluralityof subchannels), the transmission start timings of the PSDUs may beidentical (or aligned) on the plurality of subchannels. In this case,paddings may be applied individually to the plurality of subchannels,for simultaneous termination of MU transmissions on the plurality ofsubchannels in the HE PPDU.

A padding (e.g., one or more of a MAC padding and a PHY padding) may beadded based on a subchannel having the longest data unit (e.g., MPDUfield) to be transmitted among the plurality of subchannels.

For example, a padding (e.g., one or more of a MAC padding and a PHYpadding) may be added to each of one or more subchannels (i.e.,subchannels having shorter data units than the longest data unit) exceptfor the subchannel having the longest data unit among the plurality ofsubchannels. The size of the paddings (e.g., one or more of MAC paddingsand PHY paddings) added to the individual one or more subchannels may bedetermined to be values so that the lengths of the data units may beequal to the length of the longest data unit.

Or a padding (e.g., one or more of a MAC padding and a PHY padding) maybe added individually to each of the plurality of subchannels in such amanner that the lengths of data units added with paddings (e.g., one ormore of MAC paddings and PHY paddings) may be equal across the pluralityof subchannels. That is, a padding (e.g., one or more of a MAC paddingand a PHY padding) having a length exceeding 0 may also be applied to asubchannel having the longest data unit among the plurality ofsubchannels. In this case, the length of the padding applied to thesubchannel having the longest data unit may be smaller than the lengthsof the paddings (e.g., one or more of MAC paddings and PHY paddings)added to the other subchannels.

If PHY paddings of the same length are applied to the plurality ofsubchannels, a MAC padding may be added, for example, based on asubchannel having the longest MPDU field among the plurality ofsubchannels. In the example of FIG. 14, an MPDU field may correspond toone MPDU or A-MPDU. An MPDU length may be represented in units of anOFDM symbol duration (or in OFDM symbols) of an MPDU transmitted on asubchannel.

In the example of FIG. 14, since the MPDU field of a first subchannel(i.e., a subchannel on which an AP transmits an MPDU field to STA1 andSTA2) is longest, a MAC padding may be added to a subchannel having ashorter MPDU field than the longest MPDU field. For example, the lengthsof MAC paddings added to a second subchannel (i.e., a subchannel onwhich the AP transmits an MPDU field to STA3 and STA4), a thirdsubchannel (i.e., a subchannel on which the AP transmits an MPDU fieldto STA5), and a fourth subchannel (i.e., a subchannel on which the APtransmits an MPDU field to STA6) may be determined so that the lengthsof A-MPDUs having the MPDU fields and MAC paddings may be equal to thelength of the MPDU field on the first subchannel.

Also, MAC paddings may be applied to all of the plurality ofsubchannels. That is, a MAC padding having a length exceeding 0 may alsobe applied to a subchannel having the longest MPDU field among theplurality of subchannels. In this case, the MAC padding applied to thesubchannel having the longest MPDU field may be shorter than the MACpaddings applied to the other subchannels.

In addition to a MAC padding, a PHY padding may be applied to each ofthe plurality of subchannels in the HE PPDU in order to fill the codedbits of the last symbol on each of the plurality of subchannels in theHE PPDU.

In addition to the MAC padding and the PHY padding, an extension padding(e.g., a Signal Extension field) may be added to each of the pluralityof subchannels in the HE PPDU.

FIG. 15 depicts another exemplary HE PPDU padding according to thepresent invention.

The example of FIG. 15 corresponds to a case in which the transmissionstart timings of PSDUs are not identical (or aligned) on a plurality ofsubchannels in a HE PPDU format. If different HE-LTF section lengths areset for the plurality of subchannels, the transmission start timings ofthe PSDUs may be different on the plurality of subchannels. In thiscase, paddings may be applied individually to the plurality ofsubchannels to simultaneously terminate MU transmissions on theplurality of subchannels in the HE PPDU.

A padding (e.g., one or more of a MAC padding and a PHY padding) may beadded based on a subchannel having the largest sum of the lengths of adata unit (e.g., an MPDU field) to be transmitted and a HE-LTF section(refer to FIG. 7 or FIG. 8) among the plurality of subchannels.

For example, a padding (e.g., one or more of a MAC padding and a PHYpadding) may be added to each of one or more subchannels (i.e.,subchannels each having the sum of the lengths of a data unit and aHE-LTF section smaller than the largest sum) except for the subchannelhaving the largest sum of the lengths of a data unit and a HE-LTFsection among the plurality of subchannels. The size of a padding (e.g.,one or more of a MAC padding and a PHY padding) added individually toeach of the one or more subchannels may be determined so that the sum ofthe lengths of a data unit on each of the one or more subchannels may beequal to the largest sum of the lengths of a data unit and a HE-LTFsection.

Or a padding (e.g., one or more of a MAC padding and a PHY padding) maybe added individually to each of the plurality of subchannels in such amanner that the sums of the lengths of data units added with paddings(e.g., one or more of MAC paddings and PHY paddings) and HE-LTF sectionsmay be equal across the plurality of subchannels. That is, a padding(e.g., one or more of a MAC padding and a PHY padding) having a lengthexceeding 0 may also be applied to the subchannel having the largest sumof the lengths of a data unit and a HE-LTF section among the pluralityof subchannels. In this case, the length of a padding applied to thesubchannel having the largest sum of the lengths of a data unit and aHE-LTF section may be smaller than the lengths of paddings (e.g., one ormore of MAC paddings and PHY paddings) added to the other subchannels.

If PHY paddings of the same length are applied to the plurality ofsubchannels, a MAC padding may be added, for example, based on asubchannel having the largest sum of the lengths of a data unit and aHE-LTF section (refer to FIG. 7 or FIG. 8) among the plurality ofsubchannels. In the example of FIG. 15, an MPDU field may correspond toone MPDU or A-MPDU. An MPDU length may be represented in units of anOFDM symbol duration (or in OFDM symbols) of an MPDU transmitted on asubchannel. A HE-LTF length may be represented in units of an OFDMsymbol duration (or in OFDM symbols) and may correspond to thetransmission time of a Space-Time Stream (STS) training sequence.

In the example of FIG. 15, since the sum of the lengths of an MPDU fieldand a HE-LTF section is largest for the first subchannel (i.e., thesubchannel on which the AP transmits an MPDU field to STA1 and STA2), aMAC padding may be added to a subchannel having a smaller sum of thelengths of an MPDU field and a HE-LTF section than the largest sum. Forexample, the lengths of MAC paddings added to the second subchannel(i.e., the subchannel on which the AP transmits an MPDU field to STA3and STA4), the third subchannel (i.e., a subchannel on which the APtransmits an MPDU field to STA5), and the fourth subchannel (i.e., thesubchannel on which the AP transmits an MPDU field to STA6) may bedetermined so that the sums of the lengths of the MPDU fields, theHE-LTF sections, and the MAC paddings may be equal to the sum of thelengths of the MPDU field and the HE-LTF section on the firstsubchannel.

In addition to a MAC padding, a PHY padding may be applied to each ofthe plurality of subchannels in the HE PPDU in order to fill the codedbits of the last symbol on each of the plurality of subchannels in theHE PPDU.

In addition to the MAC padding and the PHY padding, an extension padding(e.g., a Signal Extension field) may be added to each of the pluralityof subchannels in the HE PPDU.

According to the HE PPDU paddings described with reference to theexamples of FIGS. 14 and 15, transmissions may end at the same timepoint on a plurality of subchannels in a DL/UL MU PPDU. That is,transmissions to a plurality of STAs may end at the same time point in aDL MU-MIMO or OFDMA MIMO PPDU according to a HE PPDU padding of thepresent invention.

FIG. 16 is a view comparing the lengths of MPDU fields of a plurality ofsubchannels in the example of FIG. 15.

Whether to apply a MAC padding to a subchannel and the size of the MACpadding may be determined based on the length of an MPDU field on eachsubchannel in the example of FIG. 14. On the other hand, whether toapply a MAC padding to a subchannel and the size of the MAC padding maybe determined based on the sum of the lengths of an MPDU field and aHE-LTF section on each subchannel in the example of FIG. 15.

As described above, while a MAC padding is not applied to a subchannelhaving the longest MPDU field among a plurality of subchannels in theexample of FIG. 14, a MAC padding may also be applied to a subchannelhaving the longest MPDU field in the example of FIG. 15, as illustratedin FIG. 16. This is because whether to apply a MAC padding and the sizeof the MAC padding are determined based on the length of a HE-LTFsection as well as the length of an MPDU field on a subchannel in theexample of FIG. 15. However, a HE PPDU may be terminated at the sametime point on a plurality of subchannels according to the HE PPDUpaddings of the present invention in both examples of FIGS. 14 and 15.

FIG. 17 depicts another exemplary HE PPDU padding according to thepresent invention.

As described before, a MAC padding, a PHY padding, an extension padding,etc. may be applied to simultaneously terminate transmissions on aplurality of subchannels in a DL/UL MU PPDU. In the foregoing examples,an A-MPDU EOF padding, that is, adding a null MPDU for a MAC padding hasbeen described. An additional exemplary MAC padding will be describedwith reference to the example of FIG. 17.

As in the example of FIG. 14 or FIG. 15, whether to apply a MAC paddingand the length of the MAC padding may be determined for each of aplurality of subchannels in a DL/UL MU PPDU. The example of FIG. 17 isbased on the assumption that a MAC padding is applied to a firstsubchannel (a subchannel on which the AP transmits a PSDU to STA1) of afirst DL MU PPDU (i.e., DL OFDMA PPDU 1). A PSDU of the first subchannelmay correspond to an A-MPDU and the A-MPDU may include A-MPDU subframe 1and A-MPDU subframe 2. A-MPDU subframe 3 may be configured to a lengthcorresponding to the size of a MAC padding applied to the firstsubchannel. That is, an A-MPDU subframe may be configured instead of aMAC padding field in the example of FIG. 14 or FIG. 15. A-MPDU subframe3 may correspond to a fragment of an original A-MPDU subframe, asdescribed later.

Specifically, if a last A-MPDU subframe is completely transmitted on asubchannel of a DL/UL MU PPDU, it may occur that an MU PPDU duration (orallowed TXTIME) set for the DL/UL MU PPDU is exceeded. In this case, thelast A-MPDU subframe of the DL/UL MU PPDU may be split into twofragments. The length of the first of the two fragments may bedetermined to be equal to the length of a MAC padding determined in theexample of FIG. 14 or FIG. 15.

For example, if a last A-MPDU subframe destined for STA1 is transmittedin DL OFDMA PPDU 1 in the example of FIG. 17 and a TXTIME set for DLOFDMA PPDU 1 is exceeded on a corresponding subchannel, the last A-MPDUsubframe may be split into two fragments. The first fragment may beincluded as A-MPDU subframe 3 in DL OFDMA PPDU 1. If a block ACK frameis received in response to DL OFDMA PPDU 1, the second fragment may beincluded as A-MPDU subframe 1 in the following DL OFDMA PPDU 2.

If an immediate ACK is received in response to the first fragment of thetwo split fragments (i.e., the split A-MPDU subframes), the secondfragment may be transmitted immediately. For example, MU PPDUs includingthe two fragments as split A-MPDU subframes may be transmittedsuccessively through an immediate ACK transmitted at an interval of anSIFS. That is, the interval between an MU PPDU including the firstfragment and an ACK PPDU (e.g., a first block ACK frame) transmitted inresponse to the MU PPDU may be an SIFS, the interval between the ACKPPDU and an MU PPDU including the second fragment may be an SIFS, andthe interval between the MU PPDU including the second fragment and anACK PPDU (e.g., a second block ACK frame) transmitted in response to theMU PPDU may be an SIFS.

Implementation of an MU PPDU exchange sequence including an MPDU splitinto fragments (e.g., an A-MPDU subframe split into fragments) may berestricted to one TXOP. If all of the MPDU fragments cannot be exchangedwithin one TXOP, the MU PPDU exchange sequence may be configured not toinclude the MPDU fragments.

Further, one HE PPDU may be configured not to carry a plurality offragments of one MPDU. That is, a plurality of A-MPDU subframe fragmentsinto which one A-MPDU subframe is split may not be allowed to beincluded together in one HE PPDU. For example, one A-MPDU subframe issplit into three A-MPDU subframe fragments and it may not be allowedthat DL OFDMA PPDU 1 includes two of the A-MPDU subframes and DL OFDMAPPDU 2 includes the other A-MPDU subframe. However, A-MPDU subframefragments split from different A-MPDU subframes may be included in oneHE PPDU.

To support transmission of these MPDU fragments (or A-MPDU subframefragments), a fragmentation subfield (or a split subfield) may bedefined in the MPDU delimiter field of an A-MPDU. If the fragmentationsubfield is set to 1, this may indicate that the A-MPDU subframe is oneof fragments split from one A-MPDU subframe. If the fragmentationsubfield is set to 0, this may indicate that the A-MPDU subframe is nota fragment.

In the example of FIG. 17, if the fragmentation subfield of A-MPDUsubframe 3 is set to 1 in DL OFDMA PPDU 1, this may indicate that A-MPDUsubframe 3 is an A-MPDU subframe fragment. Further, if the fragmentationsubfield of A-MPDU subframe 1 is set to 1 in DL OFDMA PPDU 2, this mayindicate that A-MPDU subframe 1 is an A-MPDU subframe fragment.

In the example of FIG. 17, if STA1 receives an A-MPDU subframe with afragmentation subfield set to 1 in DL OFDMA PPDU 1 and fails toimmediately receive the subsequent additional A-MPDU subframe with afragmentation field set to 1 (e.g., within the same TXOP), STA1 maydiscard the fragment of the received A-MPDU subframe (i.e., A-MPDUsubframe 3 of DL OFDMA PPDU 1). On the other hand, if STA1 successfullyreceives the second frame (i.e. A-MPDU subframe 1 of DL OFDMA PPDU 2)following the first fragment (i.e. A-MPDU subframe 3 of DL OFDMA PPDU 1)(e.g., within the same TXOP), STA1 may acquire information of theoriginal A-MPDU subframe by merging the two fragments.

In the example of FIG. 17, A-MPDU subframe 3 of DL OFDMA PPDU 2 has afragmentation subfield set to 0 and thus is not added with a MAC paddingor an MPDU fragment corresponding to the length of the MAC padding.Meanwhile, when needed, a MAC padding or an MPDU fragment correspondingto the length of the MAC padding may be added on another subchannel ofDL OFDMA PPDU 2.

While not shown in the example of FIG. 17, PHY paddings and extensionpaddings may be added to DL OFDMA PPDU 1 and a plurality of subchannelsof the DL OFDMA PPDU.

While the HE PPDU paddings of the present invention have been describedmainly in the context of a DL MU PPDU in FIGS. 14 and 15, the presentinvention is not limited thereto. According to a HE PPDU padding of thepresent invention, transmissions may end at the same time point on aplurality of subchannels in a UL MU PPDU. A trigger frame may indicatethe end time of the UL MU PPDU (or the length of the UL MU PPDU). Thatis, transmissions from a plurality of STAs may end at the same timepoint in a UL MU-MIMO or OFDMA PPDU.

Further, while an example of including an MPDU fragment instead of a MACpadding in a DL MU PPDU has been described with reference to FIG. 17,the present invention is not limited thereto. Transmissions may beconfigured to end at the same time pint on a plurality of subchannels ina UL MU PPDU by implementing MPDU fragment transmission according to thepresent invention. A trigger frame may indicate the end time of the ULMU PPDU (or the length of the UL MU PPDU). That is, transmissions from aplurality of STAs may end at the same time point in a UL MU-MIMO orOFDMA PPDU by transmitting an MPDU fragment instead of a MAC padding.

As described above, if an MPDU fragment (or an A-MPDU subframe fragment)is included in a DL/UL MU PPDU instead of a MAC padding, the ending timepoints of the DL/UL MU PPDU may be aligned on a plurality of subchannelsand resource use efficiency may be increased.

FIG. 18 illustrates an exemplary format of a trigger frame.

As illustrated in FIG. 18, a trigger frame eliciting UL MU transmissionmay include a Common Info field and a Per-User Info field.

In the Common Info field, a UL MU Duration subfield indicates thetransmission time of a UL HE PPDU in which a plurality of STAs performsimultaneous UL transmissions. In the examples of the present invention,the ending time (or length) of a UL MU PPDU may be determined based onthe value of the UL MU Duration subfield of the trigger frame.

A Total LTFs subfield indicates the number of HE-LTF symbols (e.g., thenumber of HE-LTF elements for each subchannel) included in the UL HEPPDU.

An LTF Duration subfield indicates the duration or length of a HE-LTFsymbol (e.g., the duration or length of a HE-LTF element for eachsubchannel) included in the UL HE PPDU.

A Guard Interval subfield indicates a guard interval applied to the ULHE PPDU.

In the Per-User Info field, an Association Identifier (AID) subfieldindicates an ID of an STA participating in the UL MU transmission.

An RU subchannel subfield indicates a subchannel that the STA uses inthe UL HE PPDU transmission.

An RU MCS subfield indicates an MCS that the STA uses in the UL HE PPDUtransmission.

An RU STS subfield indicates the number of STSs that the STA uses in theUL HE PPDU transmission.

An RU Beamformed subfield provides information about beamforming thatthe STA applies to the UL HE PPDU transmission.

An RU Coding subfield indicates coding (e.g., BCC or LDPC) that the STAuses in the UL HE PPDU transmission.

An RU Space-Time Block Coding (STBC) subfield indicates whether the STAwill use STBC for the UL HE PPDU transmission.

FIG. 19 is a flowchart illustrating an exemplary method according to thepresent invention.

In step S1910, an AP may determine whether to apply a padding to each ofa plurality of subchannels in order to transmit data to a plurality ofSTAs on a transmission channel divided into the plurality ofsubchannels. The determination may be made based on the length of a dataunit to be transmitted on each of the plurality of subchannels so thattransmissions may end at the same time point on the plurality ofsubchannels. For example, it may be determined not to apply a padding toa subchannel carrying a longest data unit among the plurality ofsubchannels. Or it may be determined to apply a padding to a subchannelcarrying a short data unit, relative to other subchannels.

In step S1920, the AP may determine a padding length for each of one ormore subchannels to which it is determined to apply paddings. A paddinglength may be determined individually for each subchannel in such amanner that transmissions may end at the same time point on theplurality of subchannels. Meanwhile, it may be said that the paddinglength is 0 for a subchannel to which it is determined not to apply apadding.

In step S1930, the AP may generate a PPDU (e.g., a DL MU PPDU) includinga data unit without a padding or a data unit with a padding (dependingon a determination as to whether a padding is to be applied) on each ofthe plurality of subchannels and may transmit the PPDU on thetransmission channel. Or it may be said that the AP generates a PPDU(e.g., a DL MU PPDU) including a data unit added with a padding (thelength of the padding is determined to be a value equal to or largerthan 0 for each subchannel according to whether a padding is to beapplied to the subchannel) on each subchannel and may transmit the PPDUon the transmission channel to the plurality of STAs.

While not shown in FIG. 19, upon receipt of the HE PPDU (e.g., DL MUPPDU) frame from the AP, an STA may transmit an ACK a predetermined time(e.g., an SIFS) after the reception time of the frame. According tovarious embodiments of the present invention, since a padding addedindividually to each of a plurality of subchannels in a HE PPDU does notcorrespond to an actual data transmission period (i.e., a time periodover which a DL MU PPDU receiver is supposed to receive data) on thesubchannel, the STA may generate the ACK by processing (e.g., decoding)data received during a part or whole of a padding period. In thismanner, the STA may further secure a time for transmitting the ACK thepredetermined time (e.g. SIFS) after receiving the frame by a padding inthe HE PPDU.

FIG. 20 is a flowchart illustrating another exemplary method accordingto the present invention.

In step S2010, a first STA may receive a trigger frame includinginformation required for simultaneous UL transmissions of the first STAand one or more other STAs on a plurality of subchannels (e.g.,subchannel allocation information, and UL MU transmission schedulinginformation (e.g., information about the length or ending time of a ULMU PPDU frame)).

In step S2020, the first STA may determine whether to apply a padding toa subchannel allocated to the first STA. The determination may be madebased on the length or ending time of the PPDU frame and the length of adata unit transmitted on the subchannel allocated to the first STA sothat transmissions may end at the same time point on the plurality ofsubchannels. For example, if a transmission ending time based on thelength of the data unit transmitted on the subchannel allocated to thefirst STA is earlier than the length or ending time of the PPDU frame,the STA may determine to apply a padding. Or if the transmission endingtime based on the length of the data unit transmitted on the subchannelallocated to the first STA is equal to the length or ending time of thePPDU frame, the STA may determine not to apply a padding.

In step S2030, if the first STA determines to apply a padding to thesubchannel allocated to the first STA, the first STA may determine alength for the padding. The length of the padding may be determinedbased on the length or ending time of the PPDU frame and the length ofthe data unit transmitted on the subchannel allocated to the first STAso that the transmissions may end at the same time point on theplurality of subchannels. Meanwhile, if a padding is not applied to thesubchannel allocated to the first STA, it may be said that the length ofthe padding is determined to be 0.

In step S2040, the first STA may generate a PPDU including a data unitwithout a padding or a data unit added with a padding (depending onwhether a padding is applied or not) on the subchannel allocated to thefirst STA and transmit the PPDU to the AP. Or it may be said that thefirst STA generates a PPDU including a data unit added with a padding onthe subchannel allocated to the first STA (i.e., including a padding ofa length equal to or larger than 0 depending on whether a padding isapplied to each subchannel) and transmits the PPDU to the AP.

While not shown in FIG. 20, upon receipt of the HE PPDU (e.g., the UL MUPPDU) frame from a plurality of STAs, the AP may transmit an ACK apredetermined time (e.g., SIFS) after the reception time of the frame.According to various embodiments of the present invention, since apadding added individually to each of a plurality of subchannels in a HEPPDU does not correspond to an actual data transmission period (i.e., atime period over which a UL MU PPDU receiver is supposed to receivedata) on the subchannel, the AP may generate an ACK by processing (e.g.,decoding) data received during a part or whole of a padding period. Inthis manner, the AP may further secure a time for transmitting the ACKthe predetermined time (e.g. SIFS) after receiving the frame by apadding in the HE PPDU.

While the exemplary method has been described with reference to FIGS. 19and 20 as a series of operations for simplicity of description, thisdoes not limit the sequence of steps. When needed, steps may beperformed at the same time or in a different sequence. All of theexemplary steps are not always necessary to implement the methodaccording to the present invention.

The foregoing embodiments of the present invention may be implementedindependently or one or more of the embodiments may be implementedsimultaneously, for the method of FIGS. 19 and 20.

The present invention includes an apparatus for processing or performingthe method according to the present invention (e.g., the wireless deviceand its components described with reference to FIGS. 1, 2, and 3).

The present invention includes software (an operating system (OS), anapplication, firmware, a program, etc.) for executing the methodaccording to the present invention in a device or a computer, and amedium storing the software that can be executed in a device or acomputer.

While various embodiments of the present invention have been describedin the context of an IEEE 802.11 system, they are applicable to variousmobile communication systems.

What is claimed is:
 1. A method for transmitting data to a plurality ofStations (STAs) on a transmission channel by an Access Point (AP) in aWireless Local Area Network (WLAN), the transmission channel beingdivided into a plurality of subchannels allocated to the plurality ofSTAs, the method comprising: generating a padding individually for oneor more subchannels to which paddings are applied among the plurality ofsubchannels, wherein the padding has a length such that transmissionsend simultaneously on the plurality of subchannels; and transmitting aPhysical layer Protocol Data Unit (PPDU) frame including a data unitwithout the padding or a data unit added with the padding for each ofthe plurality of subchannels to the plurality of STAs on thetransmission channel.
 2. The method according to claim 1, wherein thegenerating the padding comprises: determining whether to apply a paddingto each of the plurality of subchannels based on a starting time of thedata unit and a length of the data unit for each of the plurality ofsubchannels; and determining the length of the padding appliedindividually for the one or more subchannels based on the starting timeof the data unit and the length of the data unit for each of theplurality of subchannels.
 3. The method according to claim 1, whereinthe data unit added with the padding for each of the plurality ofsubchannels corresponds to an Aggregated MAC Protocol Data Unit(A-MPDU).
 4. The method according to claim 1, wherein the paddingincludes an A-MPDU padding, and the A-MPDU padding corresponds to one ormore 4-octet A-MPDU subframes having null data.
 5. The method accordingto claim 1, wherein the padding corresponds to one of a plurality offragments divided from one A-MPDU subframe.
 6. The method according toclaim 1, wherein the padding includes one or more of a Medium AccessControl (MAC) padding, a Physical (PHY) padding, or an extensionpadding.
 7. The method according to claim 6, wherein the lengths of theone or more of the MAC padding, the PHY padding, and the extensionpadding are determined individually for the one or more subchannels. 8.The method according to claim 7, wherein the lengths of the one or moreof the MAC padding, the PHY padding, or the extension padding aredetermined to be equal across the one or more subchannels.
 9. The methodaccording to claim 2, wherein a starting time of a data unit on one ofthe plurality of subchannels is identical to starting times of one ormore data units on one or more other subchannels among the plurality ofsubchannels.
 10. The method according to claim 9, wherein the startingtimes of the plurality of data units on the plurality of subchannels aredetermined based on a length of a High Efficiency-Long Training Field(HE-LTF) field further included in the PPDU frame, and wherein astarting time of the HE-LTF field is identical across the plurality ofsubchannels and an ending time of the HE-LTF field is identical acrossthe plurality of subchannels.
 11. A method for transmitting data to anAccess Point (AP) by a Station (STA) in a Wireless Local Area Network(WLAN), the method comprising: receiving a trigger frame from the AP,the trigger frame allocating a plurality of subchannels to the STA andone or more other STAs; when a padding is applied to a subchannelallocated to the STA, generating the padding having a length such that atransmission on the subchannel allocated to the STA and transmissions onone or more other subchannels allocated to the one or more other STAsend simultaneously; and transmitting a Physical layer Protocol Data Unit(PPDU) frame including a data unit without the padding or a data unitadded with the padding to the AP on the subchannel allocated to the STA.12. The method according to claim 11, wherein the generating the paddingcomprises: determining whether to apply a padding to the subchannelallocated to the STA based on a length of the PPDU frame indicated bythe trigger frame, a starting time of the data unit, and a length of thedata unit on the subchannel allocated to the STA; and determining thelength of the padding applied to the subchannel allocated to the STAbased on the length of the PPDU frame indicated by the trigger frame,the starting time of the data unit, and the length of the data unit onthe subchannel allocated to the STA.
 13. The method according to claim11, wherein the data unit added with the padding corresponds to anAggregated MAC Protocol Data Unit (A-MPDU).
 14. The method according toclaim 11, wherein the padding includes an A-MPDU padding, and the A-MPDUpadding corresponds to one or more 4-octet A-MPDU subframes having nulldata.
 15. The method according to claim 11, wherein the paddingcorresponds to one of a plurality of fragments divided from one A-MPDUsubframe.
 16. The method according to claim 11, wherein the paddingincludes one or more of a Medium Access Control (MAC) padding, aPhysical (PHY) padding, or an extension padding.
 17. The methodaccording to claim 16, wherein the lengths of the one or more of the MACpadding, the PHY padding, and the extension padding are determinedindividually for the plurality of subchannels.
 18. The method accordingto claim 17, wherein the lengths of the one or more of the MAC padding,the PHY padding, or the extension padding are determined to be equalacross the plurality of subchannels.
 19. The method according to claim12, wherein a starting time of the data unit on the subchannel allocatedto the STA is identical to starting times of one or more data units onthe one or more other subchannels.
 20. The method according to claim 19,wherein the starting times of the plurality of data units on theplurality of subchannels are determined based on a length of a HighEfficiency-Long Training Field (HE-LTF) field further included in thePPDU frame, and wherein a starting time of the HE-LTF field is identicalacross the plurality of subchannels and an ending time of the HE-LTFfield is identical across the plurality of subchannels.